Cancer Antigen 15-3 (CA 15-3) Tumor Marker Testing

CA 15-3 Biomarker Test - Why and When is it Used?

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What is the cancer antigen 15-3 (CA 15-3) blood test for breast cancer, and what does it mean?  What are the reasons for ordering a test for this biomarker?

Cancer Antigen 15-3 (CA 15-3) - Definition

Cancer antigen 15-3 (CA 15-3) refers to a blood test that may be performed during treatment for breast cancer.

What is CA 15-3?

CA 15-3 is a protein that is a normal product of your breast tissue, and it does not cause breast cancer.

If a cancerous tumor (cells growing out of control) is present in your breast, though, your levels of CA 15-3 may increase as the number of cancer cells increase. Tumor cells will shed copies of the CA 15-3 protein, which can be measured by this blood test and by a related test of cancer antigen 27.29 (CA 27.29).

Proteins that can be measured in the blood to monitor the growth of a cancer are referred to as tumor makers or biomarkers - they "mark" the presence of the cancer.

Normal Values of CA 15-3

Lab values may vary depending on the lab, but a normal CA 15-3 level is usually considered to be one that is 30 U/mL or less.

When is a CA 15-3 Biomarker Test Abnormal?

It's important to note that if a CA 15-3 test is elevated, it doesn't necessarily mean you have metastatic breast cancer, or even breast cancer at all.  Below are mentioned several other conditions which can cause an elevated CA 15-3 level.

On the other hand, a normal CA 15-3 test does not necessarily mean that you are responding to treatment.  Not every breast cancer causes a risk in CA 15-3.

How Common is an Elevated CA 15-3 Tumor Marker?

As noted, not every breast tumor causes a rise in CA 15-3, so for patients with tumors that do not produce CA 15-3 or with early-stage breast cancer, this test is not useful.

Only about 30% of patients with localized breast cancer (cancer limited to the breast) will have increased levels of CA 15-3. In patients with metastatic breast cancer, CA 15-3 can be found in 50 to 90% of all cases.

It is most useful in monitoring advanced breast cancer and your response to treatment if you have a cancer which overproduces CA 15-3.

What a CA 15-3 Does NOT Test For

The CA 15-3 test is not used as a screen for breast cancer, nor is it used as part of the diagnosis of breast cancer.

Reasons to Have a CA 15-3 Tumor Marker Test

There are several reasons for which a CA 15-3 test may be helpful.  These include:

  • Recurrence - If you have completed treatment for breast cancer your doctor may test your blood for CA 15-3 on a regular schedule to see if your levels of this antigen are rising or remaining steady.  Rising levels of CA 15-3 may indicate a recurrence of breast cancer.  Since other conditions can cause elevated levels of this antigen, however, the test results must be considered along with the results of imaging studies, your symptoms, and other tests.
  • To monitor treatment - Following levels of CA 15-3 may help your doctor know if your treatment is working.  It's important to note, however, that it may take 4 to 6 weeks of treatment before a change is seen in the level of CA 15-3.
  • To see if your cancer has progressed - If your CA 15-3 level doesn't respond or continues to increase, it may suggest that your cancer is worsening.  The level of CA 15-3 is correlated with the degree of spread of a breast cancer, and is often particularly high when bone and liver metastases are present.
  • To monitor other conditions which may be detected via a CA 15-3 test.

Other Conditions Which May Cause an Elevated CA 15-3

There are several conditions - both cancerous and benign - which can cause an elevation of the CA 15-3 level in your blood.  Some of these include:

  • Other cancers, including uterine cancer, lung cancer, pancreatic cancer, liver cancer, prostate cancer, and colon cancer
  • Cirrhosis of the liver
  • Tuberculosis
  • Benign breast conditions
  • Breastfeeding
  • Pregnancy
  • Lupus
  • Pelvic inflammatory disease
  • Endometriosis
  • Sarcoidosis

Coping with an Elevated CA 15-3 Level

If your CA 15-3 tumor marker is elevated, and it's it felt to be related to the spread of breast cancer, you and your doctor will look at several other factors in making decisions about further tests as well as treatment.  Some of these include the size of your tumor, other tumor marker tests, the results of imaging studies, as well as any findings from your history or physical exam.  It can be frightening looking at a lab test which is abnormal, but there are many other factors that go into determining the next step and your prognosis, beyond lab values.


Canadian Cancer Society. Cancer antigen 15-3 (CA15-3). Accessed 02/10/16.

Fejzic, H., Mujagic, S., Azabagic, S., and M. Burina. Tumor marker CA 15-3 in breast cancer patients. Acta Medica Academica. 2015. 44(1):39-46.

Shao, Y., Zianfu, S., Yaning, H., Chaojun, L., and H. Liu. Elevated Levels of Serum Tumor Markers CEA and CA15-3 are Prognostic Parameters for Different Molecular Subtypes of Breast Cancer. PLoS One. 2015. 10(7):e0133830.

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